The operation of mobile telephone systems now in existence can be segregated into an initial phase of call set up and a conversational phase which occurs after call set up has been completed. In the conversational phase, an air (radio) channel has been dedicated for the purposes of the conversation. On the other hand, in the set up phase the channel which is typically employed (at least for the mobile-to-base direction) relies on a contention scheme such as Aloha. Because the initial or set up phase occurs prior to either party initiating conversation, delays (even on the order of several hundred milliseconds) are not particularly critical since there is no danger of losing speech.
In the cross-referenced application, however, a mobile telephone service is described which does not dedicate an air (radio) channel for the duration of a conversation. Rather, an air (radio) channel is only provided for the duration of a speech spurt so that when each new speech spurt is detected, a process of channel allocation is required. Since this process of course cannot begin until the speech spurt has been detected, minimizing delays in allocating the channel are essential.
Aside from contention factors, delay can be occasioned by the phenomena of fading or shadowing. Furthermore, the fading can be either flat or frequency selective.
The published literature evidences previous suggestions for the use of dynamic channel assignment in the mobile telephone service on a speech spurt basis. See Al-Salihi, "The Application of Time Assignment Speech Interpolation (TASI) to the Radio Phone Service", from the Colloquium on Radio System Organization and Control, 1980; Sheikh, "Performance of Access Strategies for D-TASI Over Mobile Radio Fading Channels", in the Journal of the Institution of Electronic and Radio Engineers, Vol. 57, No. 6, November/December 1987; Nguyen, "Access Strategies for D-TASI Over Mobile Radio Channels", from the 36th IEEE Vehicular Technoloay Conference, 1986 and Riordon, "Demand Time-Assigned Speech Interpolation for Land Mobile Systems", from the International Conference--Mobile Radio Systems and Techniques, 1984.
While these publications mention the importance of minimizing delays and the fact that delays can be caused by contention and/or fading, they propose to use relatively conventional Aloha techniques of retransmission in the face of a lack of acknowledgement. More recently, Goodman, "Packet Reservation Multiple Access for Local Wireless Communications", from the IEEE Transactions on Communications, Vol. 37, No. 8, August 1989, proposes a scheme to eliminate the necessity for allocation requests and acknowledgements by actually transmitting the first speech spurt packet using contention protocols and operating on the basis that successful transmission of the first speech spurt packet prohibits each other terminal from using that particular slot until it becomes empty at the termination of the spurt. Goodman also describes using conventional Aloha retransmission in the absence of acknowledgement protocols in order to gain initial access.